1. Field of the Invention
The present invention relates to a measurement apparatus utilizing electrodes to measure physical properties of an aqueous fibrous solution, and particularly to a technique of measuring physical properties of wetstock in a sheetmaking machine.
2. State of the Art
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (wet stock) on a traveling mesh papermaking fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is essentially a de-watering system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendering rollers at the end of the process. Papermaking devices well known in the art are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol. III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
U.S. patent application Ser. No. 08/766,864, now U.S. Pat. No. 5,891,306 describes a sensor that measures water weight on the wire of a paper machine. The sensor detects changes in resistance of the wetstock between the electrodes in an electrode array. The resistance of the wetstock between the electrodes is dependent on the amount of water above the electrodes (i.e., the water weight) and the conductivity of the water. Since the conductivity of the water changes from time to time, the resistance measurement does not uniquely determine the amount of water unless some correction for the conductivity is provided. Consequently, the sensor also includes a separate reference cell which is designed to cancel out all affects that change the resistance between the electrodes other than the water weight. For instance, the resistance measurement is affected by changes in conductivity due to changes in the wetstock temperature or chemical composition. The reference cell electrode configuration is designed to have the same configuration as the measurement cell electrode configuration such that they have the same sensitivity to these conductivity changes. In particular, the spacing between electrodes is the same for both the reference cell and the measurement cell. The reference cell is positioned in a container (such as a bucket) outside of the sheetmaking machine having a continuous flow of white water provided from the sheetmaking machine. The white water of a sheetmaking machine is the water that is drained from the wire which is subsequently recycled. The depth of the white water on top of the reference cell in the container is fixed. Because the depth of water above the reference cell is fixed, any resistive changes detected by the reference cell are due to conductivity changes caused by properties other than water weight (i.e., chemical or temperature). Since the reference cell and measurement cells have the same sensitivity to conductivity, the changes in resistance due to changes in conductivity of the reference cell can be converted into a feedback signal which adjusts/compensates the input test signal Vin coupled to the electrode measurement array so that all resistance changes detected by the measurement cells are due to changes in water weight and not in conductivity changes due to chemical or temperature changes.
There are two main problems with this technique. First, the reference sensors within the container become dirty very quickly and give erroneous readings and hence does not provide a feedback signal that correctly compensates the Vin signal. Consequently, the measurement taken by the measurement array can provide erroneous readings. Moreover, the conductivity of the recycled water may be different than the water on the wire being measured. For example, fiber in the wetstock on the wire carries an ionic charge which may cause the water on the wire to be different than the recycled water with little or no fiber. Hence, the compensation or feedback signal provided by the reference cell may not provide an accurate compensation signal.